An image sensor converts a visual image to digital data that may be represented as a picture. The image sensor includes an array of pixels, which are unit devices for the conversion of the visual image into digital data. Digital cameras and optical imaging devices employ an image sensor. Image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) sensors. While CMOS image sensors (CIS) devices have been more recently developed compared to CCDs, CIS devices provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CIS devices have lower manufacturing cost compared with CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CIS devices. For these reasons, commercial employment of CIS devices has been steadily increasing in recent years.